fix(texdef): improve error handling, set CWD, and add `--tex` config

[maxchang3]

Fixes #407. For reproduction steps, see #407 and texdef-fontspec-repro.

Before and After

Before	After

Description

While investigating why texdef couldn’t resolve macros defined in .cls files within the same folder, I manually ran the logged texdef command and confirmed it worked as expected. However, in the extension, the spawned process exited immediately without any output.

Debugging revealed the following issues:

• Issue 1: We were listening on stdout.on('error'), but error messages from texdef are emitted via stderr.on('data'), so errors were never caught.
  https://github.com/tecosaur/LaTeX-Utilities/blob/a3265cc6827bd383eac4945ea8b914c0b4896fa0/src/providers/macroDefinitions.ts#L105-L108
• Issue 2: The spawned process did not have the correct working directory (cwd), which prevented it from accessing files in the workspace folder.
  • Based on testing, the default cwd on macOS is /, while on Windows, it defaults to the VS Code directory.
  • https://github.com/tecosaur/LaTeX-Utilities/blob/a3265cc6827bd383eac4945ea8b914c0b4896fa0/src/providers/macroDefinitions.ts#L96
• Issue 3: Some packages (e.g., fontspec) require an engine like xelatex, but texdef defaults to latex.

Summary of Changes

• Fixed error handling by switching from stdout.on('error') to stderr.on('data'), since stdout and stderr can appear simultaneously (confirmed by testing), so no early return on stderr.
• Added cwd to spawn options to ensure texdef runs in the correct context.
• Introduced a new configuration option latex-utilities.texdef.tex to set the --tex argument.

Future Improvements

• Automatically detect the correct engine based on the project context.
• Provide user feedback if texdef fails due to an incompatible engine.
• Should we cache the result of texdef?